U.S. Pat. No. 4,793,703 (Fretz, Jr.) describes a three-layer, glass-plastic, laminated, ophthalmic lens. That lens is composed of (1) an inorganic glass element, preferably photochromic glass, (2) a layer of a rigid, organic plastic and (3) an interlayer of flexible, organic adhesive, either thermosetting or thermoplastic. The plastic layer is selected to provide a coefficient of thermal expansion in the range of 200-700x10.sup.-7 /.degree. C. This alleviates severe stress conditions that tend to develop with higher expansion plastics, such as CR-39, marketed by PPG Industries, Pittsburgh, Pennsylvania.
A copending application, Ser. No. 07/325,880, was filed Mar. 20, 1989, now U.S. Pat. No. 5,084,712,in the name of E.R. Fretz, Jr. and assigned to the assignee of the present application. That application discloses an improved laminated lens wherein each of the three layers has reactive groups on its surface to provide improved bonding between the layers. The application may employ a rigid, epoxy layer described as "an anhydride cured epoxy, for example, a mixture of cycloaliphatic and aromatic epoxy resins cured by cycloaliphatic anhydrides such as hexahydro phthalic anhydride."
An epoxy resin system has many advantages as a material for the plastic element in a laminated lens. These include low coefficient of thermal expansion and a high Tg, which favors stability during thermal processing. Also, this system does not smear or flow during grinding and polishing, and permits adjustment of the refractive index to match that of the glass element.
A critical requirement in a laminated lens, as in any ophthalmic lens, is optical clarity. A major obstacle, encountered in development of an epoxy optical system, is striations in the epoxy casting. Such striations are commonplace in standard epoxy processing procedures, and normally of no particular concern. However, they cannot tolerated in an optical system.
Striations in an epoxy casting are not readily apparent to the unaided eye, but may easily be observed by a shadow graph technique. There, a point light source is projected through a casting onto a white background. The striations appear as dark, irregular lines on the background.
It is thought that the striations occur during gelation of the epoxy, and are the result of an unsteady state of thermal equilibrium. Thermal gradients in the gel produce convection lines that become frozen in place and cannot be dispersed. This effect is compounded by the exothermic cure of the epoxy, and the poor heat transfer of the glass casting molds employed.
Efforts to correct the problem by the common homogenizing practice of stirring, both manually and mechanically, were unsuccessful. Equally ineffective were such techniques as manipulating viscosity and/or reaction rate in the resin, and increasing heat transfer through the mold.